This invention relates to electronic reprographic systems that make copies in response to electrical printing signals and that have electronic screening or halftoning capabilities. In particular, this invention relates to electronic method and apparatus for screening previously screened images, i.e. rescreening.
Rescreening of the type to which the present invention is primarily directed occurs, for example, when a screened original (e.g. a printed picture or a typewritten document) is raster scanned by an electronic scanner to generate video signals representative of the original. The video signals are in turn electronically screened by combining them with screening signals to produce printing signals. A copy is made of the original by applying the printing signals to a binary graphic printer to create a copy which is a facsimile of the screened original. If the spatial frequency of the screen in the original differs from that of the electronic screen, the copy is distorted by beat frequencies related to the difference between the two spatial frequencies.
The motivation for including an electronic screen in electronic reprographic equipment is to make good quality facsimiles of continuous tone black and white or color originals such as monochrome (e.g. black and white) or polychrome (e.g. yellow, magenta and cyan) photographic prints or transparencies. The resultant copy is often referred to as a "first generation" copy. Often, a first generation copy is reproduced on the same reprographic equipment on which it was made. This copy of the copy is referred to as a "second generation" copy. (A copy made of the second generation copy is called a "third generation" copy and so on.) The second and subsequent generation copies are rescreened images.
The screen frequency in a "subsequent generation" copy is near but not equal to that of the earlier generation copy from which it is made. An electronic scanner distorts an image of an original to some degree. Distortion of the image is also caused by the printing mechanism in a particular reprographic machine due to parameters such as the spread of a liquid ink or dry toner on paper. In addition, a small magnification, either an enlargement or reduction, of about one to two percent is frequently desired in a reprographic machine and the magnification causes the spatial frequency of the copy to differ from the original even though the "original" is made on the same machine.
Many screened and continuous originals of course exist in today's society which have spatial frequencies near that of an electronic screen in reprographic machines. Examples of such originals are pictures, graphs and the like in newspapers, magazines and commercial offset press materials, e.g. a corporate annual report.